Oxazolidinones are widely used structural motifs in pharmaceutical applications and the cycloaddition of epoxides and isocyanates seems to be a convenient one-pot synthetic route to it. Expensive catalysts, reactive polar solvents, long reaction times and low chemoselectivities are common in early reports for the synthesis of oxazolidinones (M. E. Dyen and D. Swern, Chem. Rev., 67, 197, 1967). Due to these disadvantages there was the need for alternative methods for the production of oxazolidinones especially for application of oxazolidinones as structural motif in polymer applications.
EP 0 530 812 A1 discloses the preparation of thermosetting resinous compositions by the reaction of the mono-functional ethyl carbamate with mono- or poly-functional epoxides at a reaction temperature of 130° C. The formed oxazolidinone moiety is a terminal and/or pendant group, which reacts in situ with amines to form polymeric urea repeating units in a thermosetting polymer. The oxazolidinone intermediate comprises an N—H functional group, which reduces the thermal and chemical stability of the oxazolidinone intermediate. The thermosetting polymer formed in the reaction with amines does not soften or melt.
European Patent Application No. 12192611.7 relates to a method for the production of oxazolidinone and polyoxazolidinone compounds, comprising the step of reacting an isocyanate compound with an epoxide compound in the presence of a Lewis acid catalyst. An antimony catalyst (Ph4SbBr) was used as Lewis acidic catalyst, which has certain toxicity. The isocyanate has to be synthesised in a preceding step and requires additional safety measures due to the high reactivity of isocyanates. A mixture of the 5-oxazolidinone and 4-oxazolidinone regioisomers was obtained, which gives a less defined product mixture. The reaction of biscarbamates with bisepoxides in the presence of a base has not been disclosed in this patent application.
US 2013/0269985 describes the synthesis of polyoxazolidinones from bisepoxides and biscarbamates by using basic catalysts. The polyoxazolidinone is carbamate terminated which gives rise to thermal instability. The use of a chain regulator has not been disclosed.
The paper J. Polym. Sci. 4 (1966) 751-760 discloses polyoxazolidinones prepared from bisurethanes (biscarbamates) and bisepoxides. Although a high catalyst concentration of 18 mol % was used, long reaction times of 5-30 h were required. The polymers precipitated before a high molecular weight was reached and gradually decomposed at 250-300° C. The homo-polymerisation of the epoxide was observed as side reaction. The use of a chain regulator has not been disclosed.
Objective of the present invention was therefore to identify polyoxazolidinones having a higher thermostability than the already known polyoxazolidinones and which are useful as thermoplastic material. Furthermore a method for the preparation of these highly thermostable polyoxazolidinones, which avoids the use of free isocyanates, the use of toxic Lewis acid catalysts and the use of high catalyst concentrations, is an objective of the present invention.